The title Lucy really delivers. After all, this novel indeed focuses primarily on the experiences, changes, and perspective of a girl named Lucy. It's not like there's any bait and switch going on in which halfway through the novel, the story ends up being all about a guy named Fred.
Besides giving us a heads up on what we're in store for, the title ends up having a ton of significance thanks to Lucy's discussion of her name towards the end of novel (another reason why it's a good idea to read The Whole Book).
Specifically, Lucy recalls asking her mother why she was named Lucy:
I asked again, and this time under her breath she said, "I named you after Satan himself. Lucy, short for Lucifer. What a botheration from the moment you were conceived." (5.24)
Wow, what a heartwarming story.
But Lucy has a rather surprising reaction upon learning the origin of her name. She tells us:
I went from feeling burdened and old and tired to feeling light, new, clean. I was transformed from failure to triumph. It was the moment I knew who I was. (5.24)
Huh? She's actually happy about being likened to and named after Satan, that red guy with horns who's synonymous with evil? Even for Lucy, with all of her unconventional views, this seems a little weird.
But Lucy's reaction makes perfect sense once she mentions a little further on that her impression of Lucifer or Satan comes from having read John Milton's Paradise Lost, a long poem in part about Satan's banishment to hell after trying to overthrow God. As readers have frequently pointed out, Milton's Satan just happens to be one of the coolest and most interesting characters in all of literature.
It's not hard to imagine that Lucifer's attempt to resist the almighty and powerful rule of God is something with which Lucy would totally identify. Think about, for instance, Lucy's struggles against her domineering mother.
Similarly, Lucy's vociferous objection to her British education as a colonial subject (check out Lucy's "Character Analysis" for specifics on this one) is a prime example of her resistance against an oppressive power (although, ironically enough, her hero Milton is one of those dead English white guys she was forced to study).
Lucy's struggles against powerful forces in her life are given a ton of emphasis in the novel, making the title's reference to Lucifer especially apt.
Don't you just love it when one little title does so much?